1. Field of the Invention
The invention relates to crosslinkable compositions of silane-crosslinking prepolymers, to methods of preparing them and to the use thereof as adhesives and sealants, especially for bonding of substrates.
2. Description of the Related Art
Polymer systems having reactive alkoxysilyl groups have long been known. On contact with water or air humidity, these alkoxysilane-terminated polymers are capable of condensing with one another with elimination of the alkoxy groups even at room temperature. One of the most important applications of such materials is the production of adhesives, especially of elastic adhesive systems.
Thus, adhesives based on alkoxysilane-crosslinking polymers in the cured state do not just exhibit good adhesion properties on some substrates, but also very good mechanical properties, since they can be both tear-resistant and highly elastic. A further crucial advantage of silane-crosslinking systems over numerous other adhesive and sealant technologies (for example over isocyanate-crosslinking systems) is the toxicological safety of the prepolymers.
In many applications, preference is given to one-component systems (1K systems) which cure on contact with air humidity. The crucial advantage of one-component systems is particularly the very easy applicability thereof, since no mixing of various adhesive components by the user is required. Besides time/labor savings and safely obviating any dosage errors, there is also no need to process the adhesive/sealant within the usually quite narrow time window in the case of one-component systems, as is the case of multicomponent systems after two components are mixed.
A disadvantage of these systems, according to the prior art, is especially the low reactivity of the corresponding MS or SPUR polymers toward moisture, which necessitates aggressive catalysis. The corresponding mixtures, therefore, typically contain considerable amounts of toxicologically unsafe tin catalysts.
It is advantageous to use α-silane-terminated prepolymers, which have reactive alkoxysilyl groups bonded by a methylene spacer to an adjacent urethane unit. The compounds of this class are highly reactive and require neither tin catalysts nor strong acids or bases to achieve high curing rates on contact with air. Commercially available α-silane-terminated prepolymers are GENIOSIL® STP-E10 or -E30 from Wacker-Chemie AG.
A further problem with many adhesive and sealant systems based on silane-crosslinking polyethers is the fact that they often do not have the desired rheological effects. Thus, on one hand, the materials have to have good applicability e.g. they have to be sufficiently free-flowing under the action of shear forces. On the other hand, the adhesive or sealant should have maximum firmness after application, at least in the majority of applications. Thus, the material should no longer be able to flow away from the desired application site, not even when the site is a vertical joint or weld seam. Moreover, the material should also retain its shape. This is especially true of joints or relatively thick weld seams not covered, for example, by facings. The surface of the adhesive or sealant is usually brought into the desired shape directly after application by the user and, at the same time, also smoothed. Thereafter, the surface should, however, necessarily retain its shape.
In order to achieve this desired profile of properties, the adhesive or sealant must not just fundamentally have thixotropic properties, but the thixotropy-related viscosity buildup must also happen relatively fast after the action of the shear forces has ended.
This is, where in particular transparent silane-crosslinking adhesive and sealant systems have so far had significant shortcomings. It is possible to achieve thixotropic properties with transparent silane-crosslinking systems by standard methods, especially by addition of a fumed silica. However, these properties are usually not sufficiently developed, or are lost under relatively high material shear. For instance, the corresponding materials no longer have these properties after an action of shear forces even if they had sufficient firmness before the action of shear forces. If the corresponding materials are exposed to strong shear forces, for example, in the course of dispensing into or discharge from the respective container, or even as early as during their production, they often exhibit unwanted flow characteristics.
Nontransparent filled systems are less critical in this regard because they usually have quite a high filler content, but further improvements in properties are, of course, always desirable.